Oil pump

ABSTRACT

An oil pump includes: an outer rotor including internal teeth; an inner rotor including external teeth meshing with the internal teeth; a drive shaft connected to the inner rotor and configured to rotationally drive the inner rotor; and a housing including a pump chamber accommodating the inner and outer rotors. In the housing, a suction port and a discharge port are formed. The discharge port includes a first discharge port and a second discharge port. The suction port includes first and second suction ports respectively disposed on the same side as the first and second discharge ports. A pressure reducing oil passage communicating with the second discharge port and configured to reduce pressure of oil in the second discharge port is formed on the side of the second discharge port.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. § 119to Japanese Patent Application 2020-073053, filed on Apr. 15, 2020, theentire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to an oil pump.

BACKGROUND DISCUSSION

An oil pump includes an outer rotor including internal teeth, an innerrotor including external teeth, a housing accommodating the outer rotorand the inner rotor, and a drive shaft connected to the inner rotor. Asuction port through which oil is suctioned into a pump chamber and adischarge port through which the oil in the pump chamber is dischargedare formed in the housing. In the oil pump, the oil is pumped from thesuction port to the discharge port by rotating the inner rotor disposedeccentrically with respect to the outer rotor by the drive shaft. Suchan oil pump is disclosed in, for example, WO2019/208073 (Reference 1).

In the oil pump disclosed in Reference 1, the discharge port includes abag structure region communicating with the suction port with the pumpchamber sandwiched therebetween, and an open region continuous with adischarge oil passage. Here, at a time of operation of the oil pump, apositive pressure constantly acts on a side of the discharge port.Therefore, in the discharge port, a pressure of the oil in the bagstructure region is higher than a pressure of the oil in the openregion. In this way, since a pressure difference of the oil is presentin the housing, the outer rotor is pressed toward an open region side ofthe discharge port in the pump chamber, and a sliding friction(rotational resistance) of a pump rotor with respect to an inner surfaceof the housing is increased. As a result, it is difficult for the oil toflow, and vibration and noise easily occur.

In the oil pump using a motor as a drive source, when an input voltageto the motor is constant, a drive torque of the motor decreases as amotor rotation speed increases, and a motor current decreases. Here, anoil discharge amount of the electric oil pump is proportional to themotor rotation speed. Therefore, when the motor rotation speed increasesand the motor current decreases, the oil discharge amount per unitcurrent increases, and a pump efficiency increases. However, when therotational resistance of the outer rotor increases due to the outerrotor being pressed toward the open region side of the discharge portand the motor rotation speed decreases, the drive torque increases andthe motor current increases. As a result, the oil discharge amount perunit current decreases, and the pump efficiency decreases.

A need thus exists for an oil pump which is not susceptible to thedrawback mentioned above.

SUMMARY

A characteristic configuration of an oil pump according to thisdisclosure resides in that the oil pump includes: an outer rotorincluding internal teeth; an inner rotor including external teethmeshing with the internal teeth; a drive shaft connected to the innerrotor and configured to rotationally drive the inner rotor; and ahousing including a pump chamber accommodating the inner rotor and theouter rotor. In the housing, a suction port through which oil issuctioned into the pump chamber and a discharge port through which theoil in the pump chamber is discharged are formed. The discharge portincludes a first discharge port directly connected to a discharge oilpassage and a second discharge port with the pump chamber sandwichedtherebetween. The suction port includes a first suction port disposed onthe same side as the first discharge port and a second suction portdisposed on the same side as the second discharge port with the pumpchamber sandwiched therebetween. A pressure reducing oil passage thatcommunicates with the second discharge port and configured to reducepressure of the oil in the second discharge port is formed on a side ofthe second discharge port of the discharge port.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of thisdisclosure will become more apparent from the following detaileddescription considered with the reference to the accompanying drawings,wherein:

FIG. 1 is a side sectional view of an oil pump;

FIG. 2 is a cross-sectional view taken along a line II-II of FIG. 1;

FIG. 3 is a cross-sectional view taken along a line III-Ill of FIG. 1;

FIG. 4 is a graph illustrating a relationship between a dischargepressure and a current and a flow rate in an example and a comparativeexample;

FIG. 5 is a cross-sectional view of main parts of a modification of afirst embodiment;

FIG. 6 is a cross-sectional view of main parts of a second embodiment;

FIG. 7 is a cross-sectional view of main parts of a modification of thesecond embodiment;

FIG. 8 is a cross-sectional view of main parts of a third embodiment;and

FIG. 9 is a cross-sectional view of main parts of another embodiment.

DETAILED DESCRIPTION

Hereinafter, an oil pump according to embodiments disclosed here will bedescribed with reference to the drawings.

As illustrated in FIG. 1, an oil pump 1 includes a pump portion 10 and amotor portion 30.

The pump portion 10 includes a pump body 11, a pump rotor 14, and a pumpcover 18. The pump body 11 and the pump cover 18 are examples of ahousing. The pump rotor 14 includes an inner rotor 15 and an outer rotor16. The motor portion 30 is disposed adjacent to the pump portion 10.The motor portion 30 includes a motor body 31, a motor rotor 32, astator 33, and a motor cover 34. The motor body 31 is an example of thehousing.

As illustrated in FIGS. 1 and 2, the pump body 11 has a cylindricalouter shape, and an inside thereof is formed as a pump chamber 12 foraccommodating the pump rotor 14. The pump cover 18 is disposed adjacentto the pump body 11. The pump cover 18 and the pump body 11 are fastenedand integrated by a screw (not illustrated). The pump cover 18 and themotor body 31 are formed with a suction port 41 through which oil issuctioned into the pump chamber 12 and a discharge port 42 through whichthe oil in the pump chamber 12 is discharged.

The discharge port 42 includes a first discharge port 42 a and a seconddischarge port 42 b with the pump chamber 12 sandwiched therebetween.The suction port 41 includes a first suction port 41 a disposed on thesame side as the first discharge port 42 a and a second suction port 41b disposed on the same side as the second discharge port 42 b with thepump chamber 12 sandwiched therebetween. In the present embodiment, thefirst suction port 41 a and the first discharge port 42 a are formed inthe pump cover 18, and the second suction port 41 b and the seconddischarge port 42 b are formed in the motor body 31. In the pump cover18, a suction oil passage 43 extends outward from the first suction port41 a, and a discharge oil passage 44 extends outward from the firstdischarge port 42 a.

A bearing hole 19 is formed at a position eccentric from an axial centerof the pump chamber 12, and a rotating shaft 13 (an example of a driveshaft) that rotationally drives the inner rotor 15 is inserted andconnected so as to penetrate the bearing hole 19 and the inner rotor 15.The rotating shaft 13 is rotatably supported by the bearing hole 19, andthe rotating shaft 13 and the inner rotor 15 have a common axis center Xand rotate integrally.

In the motor portion 30, the motor rotor 32 is formed in a cylindricalshape, and the annular stator 33 is disposed outside the motor rotor 32.Both the motor rotor 32 and the stator 33 are coaxial with the axialcenter X. The motor rotor 32 is formed by accommodating and fixing amagnet 36 inside a cylindrical rotor yoke 35 formed by laminatingelectromagnetic steel plates, and rotates integrally with the rotatingshaft 13. The stator 33 includes a stator core 37 in whichelectromagnetic steel plates are laminated, a coil support frame 38 ofan insulator that covers teeth of the stator core 37, and a coil 39wound from above the coil support frame 38. An alternating current isapplied to the coil 39 by power supply from an external driver unit. Themotor rotor 32 is rotated by repetition of attraction and repulsionbetween the coil 39 and the magnet 36 by the alternating current, andthe inner rotor 15 is rotated accordingly.

As illustrated in FIG. 2, the pump rotor 14 is configured such thatexternal teeth 15 a formed on the inner rotor 15 and internal teeth 16 aformed on the outer rotor 16 mesh with each other, and an axial centerof the outer rotor 16 coincides with the axial center of the pumpchamber 12. When the inner rotor 15 rotates, the outer rotor 16 rotatesaround the inner rotor 15. A plurality of cells 17 whose volumeincreases and decreases with the rotation are formed between theexternal teeth 15 a of the inner rotor 15 and the internal teeth 16 a ofthe outer rotor 16. The oil is stored in the cells 17.

As illustrated in FIGS. 2 and 3, the suction ports 41 a and 41 b and thedischarge ports 42 a and 42 b are crescent-shaped grooves. The suctionports 41 a and 41 b are formed so as to communicate with the cells 17along a direction in which the volume of the cells 17 increases when thepump rotor 14 rotates, and the discharge ports 42 a and 42 b are formedso as to communicate with the cells 17 along a direction in which thevolume of the cells 17 decreases when the pump rotor 14 rotates. FIG. 3illustrates the second suction port 41 b and the second discharge port42 b.

Since the volume of the cell 17 communicating with the suction port 41increases as the pump rotor 14 rotates, negative pressure is generatedin the cells 17 and the oil is suctioned from the suction port 41.Thereafter, since the volume of the cells 17 communicating with thedischarge port 42 in a state where the oil is stored decreases as thepump rotor 14 rotates, positive pressure is generated in the cells 17and the oil is discharged toward the discharge port 42. The oildischarged to the discharge port 42 is pumped through the discharge oilpassage 44.

As illustrated in FIGS. 1 to 3, a lubrication groove 45 is formed in aninner peripheral region of the motor body 31 facing the rotating shaft13. The lubrication groove 45 communicates with the second dischargeport 42 b, and is provided to lubricate the rotating shaft 13 byallowing the oil to enter a gap between the bearing hole 19 and therotating shaft 13.

As described above, in the oil pump 1, the oil is discharged in a statewhere the pressure of the oil (hereinafter, also referred to as “oilpressure”) in the discharge port 42 is increased. Here, in the dischargeport 42, the first discharge port 42 a is connected to the discharge oilpassage 44, whereas the second discharge port 42 b communicates with thepump chamber 12 but is not connected to the discharge oil passage 44.Therefore, oil pressure in the second discharge port 42 b is higher thanoil pressure in the first discharge port 42 a. In this case, the outerrotor 16 is easily pressed toward the first discharge port 42 a by theoil pressure in the second discharge port 42 b, and friction is easilygenerated in the pump chamber 12.

Therefore, in the present embodiment, as illustrated in FIGS. 1 and 3,the motor body 31 includes a first groove portion 51 formed by extendingfrom the second discharge port 42 b to the outside. The first grooveportion 51 is provided to maintain a pressure balance of the pumpchamber 12. That is, the first groove portion 51 is a pressure reducingoil passage 50 that reduces the pressure of the oil in the seconddischarge port 42 b, and functions as a pressure balance groove. Thatis, since a part of the oil in the second discharge port 42 b flowsthrough the pressure reducing oil passage 50 (first groove portion 51),the pressure of the oil in the second discharge port 42 b can bereduced. Specifically, the oil in the second discharge port 42 b flowsout to the outside via the first groove portion 51 (the pressurereducing oil passage 50). Accordingly, since the oil pressure in thesecond discharge port 42 b is reliably reduced, the outer rotor 16 isless likely to be pressed toward a first discharge port 42 a side, andthe rotational resistance of the pump rotor 14 in the pump chamber 12can be reduced. As a result, in the oil pump 1, smooth rotation of thepump rotor 14 can be implemented, and pump efficiency can be improved.

Test Example

Using the electric oil pump (example) having the first groove portion 51(groove) and an electric oil pump (comparative example) not having thefirst groove portion 51 (groove), a driving current (current) suppliedto the motor portion 30 and a flow rate of the oil per unit timeaccompanying a change in the discharge pressure of the oil weremeasured. FIG. 4 illustrates a change in the current and the flow ratewith respect to the discharge pressure of the oil as a test result. Asillustrated in FIG. 4, it was demonstrated that the electric oil pump“with the groove” has a lower current value corresponding to thedischarge pressure and a higher flow rate, that is, higher pumpefficiency, than the electric oil pump “without the groove”.

[Modification of First Embodiment]

The above embodiment illustrates an example in which the pressurereducing oil passage 50 is formed of the first groove portion 51 formedin the motor body 31, and the pressure reducing oil passage 50 may beconstituted by forming a groove portion over two members. In amodification illustrated in FIG. 5, the first groove portion 51 formedin the motor body 31 is shortened, and a second groove portion 52communicating with the first groove portion 51 and communicating with anoutside is formed in the pump body 11. That is, in the presentmodification, the pressure reducing oil passage 50 includes the firstgroove portion 51 formed in the motor body 31 and the second grooveportion 52 formed in the pump body 11.

Second Embodiment

As illustrated in FIG. 6, in the second embodiment, as the pressurereducing oil passage 50, a third groove portion 53 is formed in themotor body 31 from the second discharge port 42 b toward a suction port(the second suction port 41 b in the present embodiment) on the sameside. The second suction port 41 b is formed in a crescent shape along aportion where the internal teeth 16 a of the outer rotor 16 and theexternal teeth 15 a of the inner rotor 15 mesh with each other as viewedin a direction along the axial center X, and one end side 41 b 1 of bothends in a longitudinal direction is formed to be wider than the otherend side 41 b 2. Specifically, in the second suction port 41 b, a widthW1 of the one end side 41 b 1 is larger than a width W2 of the other endside 41 b 2. Further, the second discharge port 42 b is formed in acrescent shape along the portion where the internal teeth 16 a of theouter rotor 16 and the external teeth 15 a of the inner rotor 15 meshwith each other as viewed in the direction along the axial center X, andone end side 42 b 1 of both ends in the longitudinal direction is formedto be wider than the other end side 42 b 2. Specifically, in the seconddischarge port 42 b, a width W3 of the one end side 42 b 1 is largerthan a width W4 of the other end side 42 b 2. The one end side 41 b 1 ofthe second suction port 41 b and the one end side 42 b 1 of the seconddischarge port 42 b face each other, and the other end side 41 b 2 ofthe second suction port 41 b and the other end side 42 b 2 of the seconddischarge port 42 b face each other.

The third groove portion 53 (pressure reducing oil passage 50)communicates with the other end side 42 b 2 of the second discharge port42 b, and is formed over the other end side 41 b 2 of the second suctionport 41 b. Further, the third groove portion 53 (pressure reducing oilpassage 50) is formed outside the second discharge port 42 b and thesecond suction port 41 b with respect to the rotating shaft 13.Therefore, oil in the second discharge port 42 b can be returned to thesecond suction port 41 b via the third groove portion 53 (pressurereducing oil passage 50). Accordingly, oil pressure in the seconddischarge port 42 b can be reliably reduced, and the oil pump 1 caneffectively use the oil in the second discharge port 42 b withoutdischarging the oil to the outside.

FIG. 6 illustrates virtual straight lines (one-dotted lines) L1 and L2obtained by connecting the axial center X of the rotating shaft 13 andboth ends 53 a and 53 b of the third groove portion 53 (pressurereducing oil passage 50). In FIG. 6, in the third groove portion 53, anend portion on a side of the suction port 41 is indicated by 53 a, andan end portion on a side of the discharge port 42 is indicated by 53 b.In the third groove portion 53, an angle θ formed by the two virtualstraight lines L1 and L2 on a side where the third groove portion 53 isformed around the axial center X is preferably 180 degrees or more. Inthe example illustrated in FIG. 6, the angle θ is 180 degrees or more.Accordingly, the third groove portion 53 can ensure a wide region inwhich the oil in the second discharge port 42 b flows. As a result, theoil pump 1 can reliably reduce the oil pressure in the second dischargeport 42 b by the third groove portion 53 (pressure reducing oil passage50).

In the present embodiment, as described above, when viewed in thedirection along the rotating shaft 13, in the second discharge port 42 bformed in the crescent shape, the one end side 42 b 1 of both ends inthe longitudinal direction is wider than the other end side 42 b 2.Here, in the oil pump 1, since the discharge port 42 increases the oilpressure to discharge the oil, the oil pressure on the other end side 42b 2 with a narrow width is more likely to increase than that on the oneend side 42 b 1 with a wide width. Therefore, in the present embodiment,the third groove portion 53 (pressure reducing oil passage 50) is formedso as to communicate with the other end side 42 b 2 having the narrowwidth of the second discharge port 42 b. Accordingly, the oil pressurein the second discharge port 42 b can be more effectively reduced.

(Modification of Second Embodiment)

As illustrated in FIG. 7, the third groove portion 53 (pressure reducingoil passage 50) may communicate with the one end side 42 b 1 of thesecond discharge port 42 b and may be formed over the one end side 41 b1 of the second suction port 41 b. Even in a modification illustrated inFIG. 7, the angle θ formed by the two virtual straight lines L1 and L2on a side where the third groove portion 53 (pressure reducing oilpassage 50) is formed around the axial center X is 180 degrees or more.Further, although not illustrated, the third groove portion 53 (pressurereducing oil passage 50) may be formed to communicate with the one endside 42 b 1 of the second discharge port 42 b and to extend over theother end side 41 b 2 of the second suction port 41 b, or may be formedto communicate with the other end side 42 b 2 of the second dischargeport 42 b and to extend over the one end side 41 b 1 of the secondsuction port 41 b.

Third Embodiment

The above embodiment illustrates an example in which the pressurereducing oil passage 50 is provided by the groove portions 51 and 52formed in the motor body 31 or the pump body 11. In a third embodiment,as illustrated in FIG. 8, the pressure reducing oil passage 50 is formedof a through hole 54 formed in the motor body 31. Since the pressurereducing oil passage 50 is formed with the through hole 54 instead ofthe groove portion, a degree of freedom of a shape and arrangement ofthe pressure reducing oil passage 50 is increased. Further, in thepresent embodiment, the motor body 31 and the pump body 11 may beintegrally formed. Accordingly, the number of components of the oil pump1 can be reduced.

Other Embodiments

(1) The above embodiments illustrate an example in which the suction oilpassage 43 and the discharge oil passage 44 are provided on the pumpcover 18. As illustrated in FIG. 9, the suction oil passage 43 and thedischarge oil passage 44 may be provided in the motor body 31. In thiscase, the first suction port 41 a and the first discharge port 42 a areformed in the motor body 31, and the second suction port 41 b and thesecond discharge port 42 b are formed in the pump cover 18. Although notillustrated, the suction oil passage 43 may be provided in one of themotor body 31 and the pump cover 18, and the discharge oil passage 44may be provided in the other of the motor body 31 and the pump cover 18.For example, when the suction oil passage 43 is provided in the motorbody 31 and the discharge oil passage 44 is provided in the pump cover18, the pump cover 18 is provided with the first suction port 41 a andthe first discharge port 42 a, the motor body 31 is provided with thesecond suction port 41 b and the second discharge port 42 b, the suctionoil passage 43 communicates with the second suction port 41 b, and thedischarge oil passage 44 communicates with the first discharge port 42a.

In an arrangement of the suction port 41 and the discharge port 42illustrated in FIG. 9, in the first to third embodiments describedabove, the first groove portion 51, the third groove portion 53, and thethrough hole 54 illustrated as the pressure reducing oil passage 50 areformed in the pump cover 18. FIG. 9 illustrates an example in which thefirst groove portion 51 is formed in the pump cover 18.

(2) The above embodiments illustrate an example in which a drive sourceof the rotating shaft 13 is an electric motor, but the drive source ofthe rotating shaft 13 is not limited to the electric motor. The drivesource of the rotating shaft 13 may be, for example, a crankshaft of aninternal combustion engine.

(3) The inner rotor 15 and the outer rotor 16 are not limited to thosehaving the shape and the support structure described in the aboveembodiments, and can be appropriately changed.

INDUSTRIAL APPLICABILITY

This disclosure can be widely used in an oil pump.

A characteristic configuration of an oil pump according to thisdisclosure resides in that the oil pump includes: an outer rotorincluding internal teeth; an inner rotor including external teethmeshing with the internal teeth; a drive shaft connected to the innerrotor and configured to rotationally drive the inner rotor; and ahousing including a pump chamber accommodating the inner rotor and theouter rotor. In the housing, a suction port through which oil issuctioned into the pump chamber and a discharge port through which theoil in the pump chamber is discharged are formed. The discharge portincludes a first discharge port directly connected to a discharge oilpassage and a second discharge port with the pump chamber sandwichedtherebetween. The suction port includes a first suction port disposed onthe same side as the first discharge port and a second suction portdisposed on the same side as the second discharge port with the pumpchamber sandwiched therebetween. A pressure reducing oil passage thatcommunicates with the second discharge port and configured to reducepressure of the oil in the second discharge port is formed on a side ofthe second discharge port of the discharge port.

In the oil pump, the oil is discharged in a state where pressure of theoil is increased in the discharge port. Here, in the discharge port, thefirst discharge port is connected to the discharge oil passage, whereasthe second discharge port communicates with the pump chamber but is notconnected to the discharge oil passage. Therefore, the pressure of theoil in the second discharge port is higher than the pressure of the oilin the first discharge port. In this case, the outer rotor at the pumprotor is easily pressed toward the first discharge port by the pressureof the oil in the second discharge port, and friction is easilygenerated in the pump chamber. Therefore, in the present configuration,the pressure reducing oil passage that communicates with the seconddischarge port and reduces pressure of oil in the second discharge portis formed on the side of the second discharge port which is a sideopposite to the first discharge port connected to the discharge oilpassage. Therefore, since a part of the oil in the second discharge portflows through the pressure reducing oil passage, the pressure of the oilin the second discharge port can be reduced. Accordingly, the outerrotor is less likely to be pressed toward the first discharge port side,and the rotational resistance of the pump rotor in the oil pump can bestably reduced. As a result, in the oil pump, smooth rotation of thepump rotor can be implemented, and pump efficiency can be improved.

Another characteristic configuration resides in that the pressurereducing oil passage is formed from the second discharge port to anoutside.

According to the present configuration, since the pressure reducing oilpassage is formed from the second discharge port to the outside, the oilin the second discharge port can flow out to the outside via thepressure reducing oil passage. Therefore, the pressure of the oil in thesecond discharge port can be reliably reduced.

Another characteristic configuration resides in that the pressurereducing oil passage is formed from the second discharge port to thesecond suction port.

According to the present configuration, since the pressure reducing oilpassage is formed from the second discharge port to the second suctionport, the oil in the second discharge port can be returned to the secondsuction port via the pressure reducing oil passage. Accordingly, thepressure of the oil in the second discharge port can be reliablyreduced, and the oil pump can effectively use the oil in the seconddischarge port without discharging the oil to the outside.

Another characteristic configuration resides in that the pressurereducing oil passage is formed outside the second discharge port and thesecond suction port with respect to the drive shaft, and an angle formedby two virtual straight lines on the side where the pressure reducingoil passage is formed when an axial center of the drive shaft isconnected to both ends of the pressure reducing oil passage by thevirtual straight lines is 180 degrees or more.

According to the present configuration, the pressure reducing oilpassage formed from the second discharge port to the second suction portis formed around the axial center of the drive shaft at an angle of 180degrees or more. Accordingly, the pressure reducing oil passage canensure a wide region in which the oil in the second discharge portflows. As a result, the oil pump can reliably reduce the pressure of theoil in the second discharge port by the pressure reducing oil passage.

Another characteristic configuration resides in that the seconddischarge port is formed in a crescent shape along a portion where theinternal teeth of the outer rotor and the external teeth of the innerrotor mesh with each other as viewed in a direction along the driveshaft, one end side of both ends in a longitudinal direction is formedto be wider than the other end side, and the pressure reducing oilpassage is formed so as to communicate with the other end side of thesecond discharge port.

According to the present configuration, when viewed in the directionalong the drive shaft, the second discharge port formed in the crescentshape is formed such that one end side of both ends in the longitudinaldirection is wider than the other end side. Here, in the oil pump, sincethe discharge port increases the pressure of the oil to discharge theoil, the pressure of the oil on the other end side with a narrow widthis more likely to increase than that on the one end side with a widewidth. Therefore, in the present configuration, the pressure reducingoil passage is formed so as to communicate with the other end side withthe narrow width of the second discharge port. Accordingly, the pressureof the oil in the second discharge port can be more effectively reduced.

Another characteristic configuration resides in that the pressurereducing oil passage is formed of a groove formed in the housing.

According to the present configuration, since the pressure reducing oilpassage is formed of the groove formed in the housing, the pressurereducing oil passage can be easily formed in the oil pump.

The principles, preferred embodiment and mode of operation of thepresent invention have been described in the foregoing specification.However, the invention which is intended to be protected is not to beconstrued as limited to the particular embodiments disclosed. Further,the embodiments described herein are to be regarded as illustrativerather than restrictive. Variations and changes may be made by others,and equivalents employed, without departing from the spirit of thepresent invention. Accordingly, it is expressly intended that all suchvariations, changes and equivalents which fall within the spirit andscope of the present invention as defined in the claims, be embracedthereby.

What is claimed is:
 1. An oil pump comprising: an outer rotor includinginternal teeth; an inner rotor including external teeth meshing with theinternal teeth; a drive shaft connected to the inner rotor andconfigured to rotationally drive the inner rotor; and a housingincluding a pump chamber accommodating the inner rotor and the outerrotor, wherein in the housing, a suction port through which oil issuctioned into the pump chamber and a discharge port through which theoil in the pump chamber is discharged are formed, the discharge portincludes a first discharge port directly connected to a discharge oilpassage and a second discharge port with the pump chamber sandwichedtherebetween, the suction port includes a first suction port disposed onthe same side as the first discharge port and a second suction portdisposed on the same side as the second discharge port with the pumpchamber sandwiched therebetween, and a pressure reducing oil passagecommunicating with the second discharge port and configured to reducepressure of the oil in the second discharge port is formed on the sideof the second discharge port of the discharge port.
 2. The oil pumpaccording to claim 1, wherein the pressure reducing oil passage isformed from the second discharge port to an outside.
 3. The oil pumpaccording to claim 1, wherein the pressure reducing oil passage isformed from the second discharge port to the second suction port.
 4. Theoil pump according to claim 3, wherein the pressure reducing oil passageis formed outside the second discharge port and the second suction portwith respect to the drive shaft, and an angle formed by two virtualstraight lines on the side where the pressure reducing oil passage isformed when an axial center of the drive shaft is connected to both endsof the pressure reducing oil passage by the virtual straight lines is180 degrees or more.
 5. The oil pump according to according to claim 1,wherein the second discharge port is formed in a crescent shape along aportion where the internal teeth of the outer rotor and the externalteeth of the inner rotor mesh with each other as viewed in a directionalong the drive shaft, and one end side of both ends in a longitudinaldirection is formed to be wider than the other end side, and thepressure reducing oil passage is formed so as to communicate with theother end side of the second discharge port.
 6. The oil pump accordingto claim 1, wherein the pressure reducing oil passage is formed of agroove formed in the housing.